Tensions are once again running high between the Constitutional Democratic Party of Japan and the Japanese Communist Party. After the electoral damage suffered by the CDP in the Lower House election last October, when the party fielded unified candidates with the JCP, many question the logic behind trying the strategy again in this summer's Upper House election.

And with less than six months to go until the election, both opposition parties want to prevent the pro-constitutional revision ruling coalition and like-minded opposition parties from winning the necessary seats in the Upper House by splitting the anti-revision vote. But there appears to be no prospect for a compromise.

According to CDP leader Kenta Izumi, the idea doesn't seem worth revisiting. “It did not bear fruit," he said during a Fuji TV program on Jan. 31. "At this point, we’ve made it clear that we need to go back to scratch on what to do with the (electoral) cooperation.”

“We will campaign by emphasizing our own policies, targeting the votes and the voters that we need to go after,” said Izumi.

Izumi’s comments indicated he would scrap the cooperation this time around, though he has yet to formally respond to JCP requests for talks on an agreement for the Upper House election, which must be held by July.

Izumi’s remarks and silence on intraparty talks prompted a response from JCP leader Kazuo Shii, who has been pushing the CDP to continue the arrangement.

“It’s not valid for the (CDP) to argue that a formal agreement between public political parties be unilaterally scrapped without consultation,” Shii posted on Twitter on Wednesday.

During the Lower House election in October, the CDP and the JCP fielded unified candidates in district seats aimed at preventing pro-constitutional revision parties from winning the needed two-thirds majority in the Lower House. The JCP had also agreed not to seek Cabinet posts if the election resulted in the CDP becoming the major ruling party.

The results were disastrous, however — particularly for the CDP. The party had 110 seats in the 461 seat chamber going into the election but ended up with only 96 seats, partly because voters were turned off by the cooperation with the JCP in many districts.

Then-CDP leader Yukio Edano, who had brokered the deal, was replaced by Izumi, who has been less enthusiastic about similar cooperation in the future, despite the JCP’s eagerness.

Izumi’s hesitancy was reflected in the party’s postmortem report on the Lower House election.

“Although the CDP was able to increase a certain amount of seats in single-seat districts, it failed to lead to results as much as we had hoped,” according to the report unveiled on Jan. 27.

“We agreed with the JCP on limited cooperation from outside the Cabinet on the part of the JCP (if it became a ruling party),” said Seiji Ohsaka, the party’s executive deputy president, on the same day. “But this was misunderstood by the voters, despite the fact that our agreement was not to become a ruling coalition.”

The CDP concluded that if it is to win over more independents and conservatives, it had to solidify its core constituency while broadening its outreach to attract more middle-of-the-road voters.

Pressure to avoid cooperation with the JCP is also coming from the Japanese Trade Union Confederation, known as Rengo, which is the party’s top vote-gathering machine. Top Rengo officials, including president Tomoko Yoshino, have repeatedly said that a unified candidate agreement with the JCP is impossible.

One issue that the two parties share is their opposition to revising the Constitution along the lines proposed by the ruling Liberal Democratic Party and Komeito coalition. The opposition parties Nippon Ishin no Kai and the Democratic Party for the People both advocate for a constitutional revision, as well.

After the Lower House election last year, pro-revision parties controlled more than the two-thirds majority needed to hold a referendum for a constitutional revision. However, the pro-revision camp remains short of the two-thirds it would need in the Upper House.